Hello, first post here so thanks for accepting me 🙂
Recently, a friend of mine just got a powered mixer from soundtech, model 6150 that burnt one of its speakers. We've made some initial measurements and found that the positive DC supply voltage is in its output. At first I thought it could be a shorted blocking capacitor or something but later I found that the amp was DC coupled so no caps at the end of it.
We eventually found the schematics and it seems that transistor Q104 is heating up a lot (schematics attached) but I'm not sure about the function of transistor 103 and 102 (the ones driving Q104) so I don't know what to expect from it's behavior.
I really appreciate any insight ^^
Recently, a friend of mine just got a powered mixer from soundtech, model 6150 that burnt one of its speakers. We've made some initial measurements and found that the positive DC supply voltage is in its output. At first I thought it could be a shorted blocking capacitor or something but later I found that the amp was DC coupled so no caps at the end of it.
We eventually found the schematics and it seems that transistor Q104 is heating up a lot (schematics attached) but I'm not sure about the function of transistor 103 and 102 (the ones driving Q104) so I don't know what to expect from it's behavior.
I really appreciate any insight ^^
Q104 is intended to set the idle current of the output transistors.
It should - if everything was working - have a almost constant voltage difference between collector and emitter of around 0.6V*(R111+R110)/R111 = 1.8V.
It is not intended to get hot by itself (in many cases the transistor here has been mounted in thermal contact with the heatsink for the output transistors - but that is not always so).
The Q103 and especially Q102 form the voltage amplifier (usually called the pre-driver stage) which provides the full output voltage sving. Q106, Q108 and Q111-Q116 basically is just a times one amplifier that converts the high impedance, low current output from the pre-driver to the high current drawn by the speaker.
The collector of Q102 should have a voltage of ca. -1V compared to ground - but with your failure I would expect it to be almost at the positive supply.
A possible cause for measuring the positive DC supply voltage on the output could be one of Q106 or Q111-Q113 shorting internally.
I would start having a look at these transistors (making transistor measurements when "in-circuit" is hard to do in a precise manner - but shorts are usually easy to spot).
Do remember to switch the amplifier OFF first - or you may easily fry your multimeter.
I notice that there are 2 fuses between the power supply and the amplifier: exchanging these with some power resistors - say 100 ohms when you continue to work on the repair is a time honourned method to avoid expensive disasters (for example finding out that a transistor has died, exchange it just to have it blown up again by another dead transistor, that you hadn't found yet)
You may want to visit Rod Elliots site - Elliott Sound Products - The Audio Pages (Main Index) - particulary the feature article "Amplifier Troubleshooting and Repair Guide".
Welcome to the world of making things work (again 🙂
Cheers, Martin
It should - if everything was working - have a almost constant voltage difference between collector and emitter of around 0.6V*(R111+R110)/R111 = 1.8V.
It is not intended to get hot by itself (in many cases the transistor here has been mounted in thermal contact with the heatsink for the output transistors - but that is not always so).
The Q103 and especially Q102 form the voltage amplifier (usually called the pre-driver stage) which provides the full output voltage sving. Q106, Q108 and Q111-Q116 basically is just a times one amplifier that converts the high impedance, low current output from the pre-driver to the high current drawn by the speaker.
The collector of Q102 should have a voltage of ca. -1V compared to ground - but with your failure I would expect it to be almost at the positive supply.
A possible cause for measuring the positive DC supply voltage on the output could be one of Q106 or Q111-Q113 shorting internally.
I would start having a look at these transistors (making transistor measurements when "in-circuit" is hard to do in a precise manner - but shorts are usually easy to spot).
Do remember to switch the amplifier OFF first - or you may easily fry your multimeter.
I notice that there are 2 fuses between the power supply and the amplifier: exchanging these with some power resistors - say 100 ohms when you continue to work on the repair is a time honourned method to avoid expensive disasters (for example finding out that a transistor has died, exchange it just to have it blown up again by another dead transistor, that you hadn't found yet)
You may want to visit Rod Elliots site - Elliott Sound Products - The Audio Pages (Main Index) - particulary the feature article "Amplifier Troubleshooting and Repair Guide".
Welcome to the world of making things work (again 🙂
Cheers, Martin
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